The sintering of unfluxed raw batch materials having a cordierite composition to full density is very difficult to achieve due to the narrow firing temperature range between the point where a porous sintered body is attained and the point where a body becomes overfired with resultant melting, bloating and/or other deformation. When fairly high density can be achieved, it is at temperature dangerously close to overfiring, rendering commercial production of good ware unreliable even under customary controlled furnace temperature variation.
As used in this application:
(a) "full density" and "impervious" mean the condition of a ceramic body whereby it exhibits less than 1% by volume of open porosity as determined either by the conventional mercury porosimetry test or by the boiling water test for apparent porosity generally as defined in ASTM Designation C20-70 effective Jan. 22, 1970, both of which give essentially the same results for products of the invention stated herein; PA1 (b) "raw" means the condition of ceramic batch material which is not prereacted with another batch ingredient, but which may have been individually calcined or fired without melting thereof or otherwise is unfired; PA1 (c) "prereacted" means the condition of ceramic batch material which has been formed by reaction between two or more raw materials with, at most, melting of only minor portions thereof; and PA1 (d) "mineral batch composition" means a ceramic batch composition in which all of the ceramic material is raw and/or prereacted. PA1 (a) wholly raw ceramic material wherein RO comprises, as mole % of RO, about 55-95% MnO and 5-45% MgO, and PA1 (b) at least about 50 wt. % prereacted cordierite material and the balance thereof being raw ceramic material, and wherein RO comprises, as mole % of RO, about 5-40% MnO and 60-95% MgO.
Unfluxed sintered cordierite bodies made from raw ceramic batch material commonly have significant open porosity (see U.S. Pat. No. 3,950,175, column 2, lines 13-26), but they also have higher melting points and use temperatures. Such bodies have been proposed in which as much as 98% by weight of the MgO can be replaced by MnO in the raw batch (see U.S. Pat. No. 3,885,977, column 2, lines 55-63).
It was earlier proposed in British patent specification No. 282,404 to add small amounts, such as 4-8 wt. %, of manganese oxide to MgO--Al.sub.2 O.sub.3 --SiO.sub.2 raw batches fired to form ceramic bodies. The manganese oxide was said to prevent or minimize formation of vitreous amorphous phase and to promote only the microcrystalline phases as noted in the preceding British patent specification No. 282,403, viz. a phase like sillimanite or mullite and a magnesium silicate phase like enstatite or forsterite. Such bodies were noted for their coefficients of thermal expansion being lower than those of steatite type ceramics. No indication is given about the presence or absence of impervious condition in these bodies, particularly those modified with manganese oxide.
However, German Pat. No. 611,854 asserts that certain MgO--Al.sub.2 O.sub.3 --SiO.sub.2 raw batches can be fired dense without having to add fluxing substances. This patent also indicates that some small amount of a forth component of iron oxide or other heavy metal oxide can be added, e.g. as by being an impurity in the clay of the batches. However, no clear indication is given of the presence or absence of full density in the resultant bodies without deformation thereof.
German Pat. No. 679,917 appears to suggest a new product application of the four component composition of German Pat. No. 611,854. For unglazed bodies, iron oxide may range between 2-6 wt. %. More iron oxide can be used, up to 15 wt. %, in glazed bodies.
British patent specification No. 888,227 discloses that densely vitrified cordierite bodies can be made from raw ceramic batches with a wider firing range by the required addition of lithium aluminum silicate flux and the optional further addition of iron oxide and/or manganese oxide. In the absence of the lithium aluminum silicate flux, it is noted that feldspar is a necessary flux in combination with the iron oxide and/or manganese oxide additions, but it causes an undesirable increase in coefficient of thermal expansion.
Full density is also difficult to attain in firing some sinterable/crystallizable glass powders (formed by fully melting raw ceramic batch materials) to glass-ceramic products, as noted in U.S. Pat. No. 4,191,583. That patent describes impervious, sintered, Mn--Mg cordierite glass-ceramic products made from crystallizable glass powders or cerammed powders, but it gives no indication about attaining or not attaining full density in sintered cordierite products resulting from firing mineral batch compositions.
Holland et al., J. Australian Ceram. Soc., 12(2), pp. 37-40 (1976) point out the difficulty of forming manganese cordierite (2MnO.2Al.sub.2 O.sub.3.5SiO.sub.2) by firing raw batch materials, except by repeated overnight firings at 1130.degree.-1140.degree. C. No indication is given that such fired products had full density.